The present invention relates to the field of negative electrodes for rechargeable batteries, and more particularly to binders and the carbon materials contained in the negative electrodes of rechargeable batteries using non-aqueous electrolyte.
The high energy density and high power of rechargeable batteries that use non-aqueous electrolytic solutions, particularly rechargeable lithium batteries, have made them a growing focus of research interest.
As for the active material of positive electrodes for rechargeable lithium batteries, lithium containing transition metal oxides such as LiCoO2 and LiNiO2, and chalcogen compounds including MoS2 have been studied. These compounds have a layered crystal structure, into which lithium ions may be reversibly intercalated and deintercalated. As for the active material of the negative electrode, on the other hand, the use of lithium metal has been studied. However, lithium metal results in formation of dendritic lithium on the surface of lithium because of repeated dissolution and deposition of lithium during charge and discharge. The formation of dendritic lithium decreases the charge and discharge efficiency. In addition, dendritic lithium on the negative electrode causes internal shortcircuiting by contacting the positive electrode.
To avoid these disadvantages, the use of lithium alloys, metal powder, graphite and non-crystallized carbon material, metal oxides, and metal sulfides as the negative electrode material has been studied with the aim of identifying a material which would enable the reversible intercalation and deintercalation of lithium ions.
A cylindrical battery using a negative electrode made of lithium alloy processed into a sheet undergoes fragmentation of the sheet alloy, caused by repeated and intensive charging and discharging, thus reducing its collecting capability, and resulting in degradation of its charge and discharge cycle characteristics.
A sheet electrode made of metal powder, carbon material, metal oxide, or metal sulfide requires the addition of a binder because an electrode cannot be formed only of these materials. For example, the Japanese Laid-open Patent No. H4-255670 discloses a method for forming a negative electrode from a carbon material by adding an elastic rubber polymer material as a binder. When using a metal oxide or metal sulfide for the negative electrode, a conductive material is further added in addition to the binder to achieve better charge and discharge characteristics.
In general, when the carbon material is used as the negative electrode, it is first ground into powder and then is formed into an electrode plate using the binder. The conventional method of using a rubber polymer material as a binder, however, causes the binder to coat graphite particles and interfere with the intercalation and deintercalation of lithium, resulting in a significant decrease in the otherwise high efficiency discharge characteristics of the battery, particularly at low temperatures. If graphite which has a high degree of crystallization is used as the carbon material, a battery with higher capacity and voltage is achievable compared to one using the carbon material. However, the graphite materials fragment into flakes when ground. If the negative electrode sheet is made of such ground material, the flat surfaces of graphite flakes not involved in the deintercalation align themselves parallel to the electrode plate face, resulting in a significant decrease in the otherwise high efficiency discharge characteristics of the battery, particularly at low temperatures.
If conventional binders are used, a large quantity of binder needs to be added, regardless of the type of carbon used, because conventional binders have weak adhesion to metal core of the electrode. This causes the binder to cover the surface of the carbon material, degrading the battery""s otherwise high efficiency discharge characteristics. On the other hand, a smaller amount of binder has lower adhesion, and increases the defect rate in the manufacturing process of rechargeable batteries, including the peeling of the electrode material from the core material. Accordingly, satisfactory characteristics have not yet been achieved in rechargeable lithium batteries that use carbon material for the negative electrode.
The present invention relates to the manufacture of the negative electrode for rechargeable batteries using non-aqueous electrolyte containing carbon material which intercalates and deintercalates lithium and binder. A mixture of Binder (A) selected from styrene butadiene copolymer containing styrene at between 20% and 70% and Binder (B) selected from at least one of styrene butadiene copolymer containing styrene at between 80% and 100%, and polystyrene, as a binder for a negative electrode material is used as the binder in the negative electrode material of the present invention.
The rechargeable batteries using non-aqueous electrolyte of the present invention comprises a negative electrode sheet made by applying the negative electrode materials as configured above onto a collector made of a copper foil, a positive electrode plate using lithium containing composite oxide as an active material, and non-aqueous electrolyte. Further, the non-aqueous electrolyte contains ethylene carbonate and alkyl carbonates.